Conventionally, there have been provided systems such as brake-by-wire systems, steer-by-wire systems, shift-by-wire systems and drive-by-wire systems. In such systems when a driver conducts operations such as braking, steering, shifting and accelerating, an electrical signal is generated based on the control input of the operation. The displacement amount of the actuator on the vehicle operation is determined based on the electrical signal, and braking, steering, shifting, throttle control and the like are then conducted. Taking brake-by-wire systems as an example, the one is widely known which brakes the wheels by generating an electrical signal in response to a state in which a brake pedal is operated by a driver, that is, brake pedal stroke or brake pedal leg-power and by driving the actuator based on the electrical signal. For driving the actuator, such methods are conceivable in which for each wheel a brake friction pad is directly pressed against a disk using a motor or the like to obtain wheel braking force, and in which wheel cylinder pressure is generated by a pump to obtain wheel braking force.
In the above system, in which an actuator on the conveyance operation is driven based on an electrical signal to make adjustments such as acceleration/deceleration and movement direction of the conveyance, if an electrical signal stops, the driver's intention is not transmitted to the actuator. Therefore, a system structure such that an electrical signal is always transmitted correctly is necessary. Particularly, regarding power supply from an alternator or a battery, a device structure which can take measures against interruption of power supply to the electronic control system while the vehicle is traveling is desirable.
In response to this, for example, vehicle systems which are provided with two power sources to increase reliability of power supply are disclosed in JP H10-76925A and JP 2001-114039A. In such systems, even in the case when power supply from either power source stops, since power supply from the other power source is ensured, effective measures can be taken against interruption of power supply while the vehicle is traveling.
In such devices, however, in which a storage battery is employed as a preliminarily employed power source, since the storage battery anytime has to be charged at a level where discharge is possible, problems occur such that electric power is uselessly consumed and that deterioration of performance of the storage battery progresses due to constant charge.
The present invention is conducted to solve the above problems.